1. Technical Field of the Invention
The present invention relates in general to the telecommunications field and, in particular, to the dynamic selection of echo cancellers and echo cancellation parameters in telephony systems.
2. Description of Related Art
Speech quality has become a highly competitive factor in marketing telephony systems. Echo, which is a phenomenon typically caused by imperfect impedance matching of network transmission sections, has a significant impact on the overall speech quality of telephony systems. Specifically, the primary cause of echo in many telephony systems is the impedance mismatch between connected two-wire and four-wire network transmission sections.
For example, FIG. 1 is a schematic block diagram that illustrates how echo can occur in a Public Switched Telephone Network (PSTN). As shown in a typical arrangement, a PSTN subscriber's telephone 1 is connected by a two-wire transmission line to a hybrid interface circuit 1'. The hybrid circuit (1') functions to convert from the telephone's two-wire access line to the PSTN's four-wire transmission line (needed for two-way communications). Similarly, a second PSTN subscriber's telephone 2 is connected by a two-wire transmission line to a second hybrid circuit 2', which also makes the two-wire to four-wire conversion. Since each hybrid is a non-ideal circuit, an impedance mismatch occurs at each hybrid (1', 2') between the two-wire lines and four-wire line, which reflects a portion of the incoming speech energy from the four-wire line back to each of the respective subscriber's phones (1, 2). This reflected speech energy, or echo, is typically a distorted and delayed replica of the outgoing speech reflected from the other subscriber side. In FIG. 1, the respective energy reflections (echos) for subscriber's 1 and 2 are illustrated by the dotted arrows 1" and 2".
In general, as long as the reflected energy transmission delay time is relatively short (e.g., .ltoreq.25 ms), the reflected speech energy is normally not perceived to be annoying. However, when the delay is increased beyond 25 ms, the reflected energy is normally perceived as an annoying "talker echo". The higher the delay time, the more annoying and/or confusing the echo becomes. For example, FIG. 2 is a graph that illustrates the required echo attenuation needed as a function of transmission delay time. As shown, the total echo return loss (ERL) can be plotted as a function of delay time to generate an echo tolerance curve. As the delay increases above 25 ms, echo control procedures are required.
In summary, echos become more annoying as network transmission path delay times are increased. Such transmission path delays can be introduced in a number of ways, such as by propagating speech signals over long distances, or by the process of coding transmitted speech signals. For example, significant propagation delays occur during satellite communication transmissions for intra- and intercontinental calls. A geostationary satellite (due to its substantial distance from the earth) typically introduces a one-way transmission path delay of about 260 ms, or a total (round-trip) echo path delay of about 520 ms. FIG. 3 is a diagram that shows a satellite communications link and illustrates how such a system introduces a significant transmission (echo) path delay.
Digital cellular communications systems also introduce significant echo path delays. For example, the speech and channel coding used for fault tolerance in digital cellular communications system radio transmissions introduces a one-way transmission path delay of about 100 ms, as blocks of speech samples are transmitted and re-transmitted over the air interface. FIG. 4 is a diagram that shows a digital cellular communications system and illustrates such an echo path delay.
Echo cancellers are electronic devices that are used to suppress the effects of echos in telephony systems. For example, for connections with long transmission delays between subscribers (e.g., FIG. 1), two echo cancellers are typically used (one on either side of the transmission path that causes the delay). For long distance satellite communications systems (e.g., FIG. 3) that link to PSTNs, an echo canceller is typically located at each of the gateway switches or transit nodes (e.g., at the international switching centers or ISCs) associated with each local exchange, but "facing" the respective PSTN. In a digital cellular communications system (e.g., FIG. 4) that links to a PSTN, an echo canceller is typically located at a mobile services switching center (MSC) and also "facing" the PSTN.
FIG. 5 is a simplified schematic block diagram that illustrates two echo canceller configurations that can be used in existing telephony systems: (1) trunk echo cancellers, and (2) echo cancellers in a pool (ECP) configuration. In the examples illustrated by FIG. 5, three trunk lines (2, 4, 8) are shown, each of which connects a PSTN (11) transmission line to a digital cellular system 12 via a group switch (e.g., part of an MSC) 10 for speech communications therebetween. Group switch 10 can be a component of a digital switching system, such as, for example, an AXE 10 digital switching system manufactured by Ericsson Telecom AB. An echo canceller device 14 is physically connected to trunk line 2. The electrical parameter settings for trunk echo canceller 14 are controlled by echo canceller control circuit 16. An exchange terminal circuit 18 is connected between trunk echo canceller 14 and group switch 10.
An echo canceller pool 30 is associated with group switch 10. A plurality of echo cancellers are maintained in the pool (30) to form a part of the trunk signalling subsystem (TSS) of the digital switching system. A conventional echo canceller pool 30 can be, for example, an ECP 101 product, which is manufactured by Ericsson Telecom AB. As shown, no trunk echo canceller is physically located at trunk 8. Instead, when a call is initiated between the PSTN (11) and the digital cellular system (12), the traffic control subsystem (TCS) of the digital switching system selects one of the echo cancellers in the pool (30) and directs switch 10 to route the connection through the selected echo canceller for the duration of the call. In that way, a more efficient use of echo cancellers is possible and thus reduces the total number of echo cancellers needed, as compared to prior trunk echo canceller configurations. Also, there is increased reliability over the prior systems, because there are always some echo cancellers available in the pool, which can be used in the event one or more of the other echo cancellers develops a fault.
Although the use of echo canceller pool configurations has increased the flexibility and reliability of existing telephony systems, a significant problem still exists with respect to echo cancellation. Specifically, all of the echo cancellers in each existing pool are configured with the same cancellation parameters, which significantly limits the flexibility of existing telephony systems.